MeasureIt won't show actual size of my model, only when the Subdivision Surface modifier is applied
The accepted solution, provided by batFINGER. You add an empty to the scene, by either hovering over Add in the 3D View menu, selecting Empty -> Plain Axes, or, you can alternatively, hold Shift+A, and select Empty -> Plain Axes.

Then, a tip is to use the snap function (you find it in the 3D View menu as well, or, by simply pressing Shift+Tab. Then select what you want to snap to. In my case, it was vertices. But you can snap to either vertices, edges, faces or even entire volumes. Or just incremental snapping.

You can choose what to snap to by pressing Shift+Ctrl+Tab to toggle between the different snap modes.